Solid most commonly refers to a phase (or state) of matter. Solid may also refer to:

Polyhedron, a geometric object with flat faces and straight edges

Solid Snake

Solid geometry, the traditional name for the geometry of three-dimensional Euclidean space

Solid (billiard ball), the wholly colored balls 1-7

"Solid (song)", 1984 Ashford & Simpson song

Solid (KDE), the device framework of KDE 4

A material (especially a metal) of which an object is solely made throughout (without joins or holes), e.g.: solid wood

An object without holes (cf. hollow)

Solid (album), U.D.O.'s fifth album

A material with high stiffness or strength, informally

An abbreviation for five basic class design principles in Object-Oriented-Design: Single responsibility principle, Open/closed principle, Liskov substitution principle, Interface segregation principle, Dependency inversion principle